dynamo, after which they can supply full currents for one hundred
hours--enough to take them around the globe--while partly
consuming the elements in the cells. The power is applied
through turbine screws, half of which are capable of propelling
the flat deck in its inclined position at sufficient speed to
prevent its falling. The moving parts have ball bearings and
friction rollers, lubrication being secured automatically, when
required, by a supply of vaseline that melts if any part becomes
hot. All the framing is of thin but very durable galvanized
aluminum, which has superseded steel for every purpose in which
weight is not an advantage, as in the permanent way on railways.
The air ships, whose length varies from fifty to five hundred
feet, have rudders for giving a vertical or a horizontal motion,
and several strengthening keels that prevent leeway when turning.
They are entirely on the principle of birds, maintaining
themselves mechanically, and differing thus from the unwieldy
balloon. Starting as if on a circular railway, against the wind,
they rise to a considerable height, and then, shutting off the
batteries, coast down the aerial slope at a rate that sometimes
touches five hundred miles an hour. When near the ground the
helmsman directs the prow upward, and, again turning on full
current, rushes up the slope at a speed that far exceeds the
eagle's, each drop of two miles serving to take the machine
twenty or thirty; though, if the pilot does not wish to soar, or
if there is a fair wind at a given height, he can remain in that
stratum of the atmosphere by moving horizontally. He can also
maintain his elevation when moving very slowly, and though the
headway be entirely stopped, the descent is gradual on account of
the aeroplane's great spread, the batteries and motors being
secured to the under side of the deck.